hello there...
Any form or self-harming behaviour, whether enacted by getting lost in alcohol, drugs, sex, porn, violence...or food...Is always used to stop, fill and/or negate the black hole of craving within....
Cutting would seem to be the way some cope with lingering distress, deep anger and painful memories...
And if we add in that they're not enlightened beings, grown ups and let's politely say fully functioning adults... then it's more than easy for them to throw the responsibility of themselves onto others...
A good strategy might be to help your g/f realise she's in no way responsible for this other woman's grip on reality or not...and her self-harming patterns...
Then maybe offer a link to her that will help her gain some quality therapy, the contact with her, until she's well again...